The present invention relates generally to bus switches, and more particularly to CMOS bus switches.
The digital electronics industry is migrating to lower operating voltages, but systems are performing at increasing speeds. High-speed bus switches are needed in these systems to keep the data moving fast. However, systems running at higher frequencies generate more noise and this could lead to corruption of data if not properly handled.
There is desired an improved high-speed bus switch that provides undershoot protection for the bus switch to ensure that the buses remain isolated when the switch is disabled. This would eliminate the need for special undershoot protection circuitry in the system design which results in a less expensive system.
A basic CMOS crossbar switch is shown in FIG. 1 as circuit 10. This circuit 10 is typical of the input/output configuration of a bus switch, which is an N-channel pass transistor MN1 with its source and drain connected to two different buses A and B. The gate of the pass transistor MN1 is controlled by an output enable (OE) signal generated from an output of an enable circuit. Thus, the switch MN1 will be closed when the gate voltage of transistor MN1 is high, and the switch MN1 will be open when the gate voltage of transistor MN1 is low. Also included in FIG. 1 are two Schottky diodes, D1 and D2, that are used for undershoot protection. In the enabled state (gate voltage of transistor MN1 is high), the undershoot voltage is not a problem. However, in the disabled state (gate voltage of transistor MN1 is low), a negative voltage on a bus could cause the N-channel pass transistor MN1 to turn on. This occurs when the negative bus voltage is larger in magnitude than the Vtn of transistor MN1. When one of the buses has a negative voltage that exceeds the forward turn-on voltage of the Schottky diode, then the diode will turn on and clamp the source or drain voltage of transistor MN1 in order to keep the buses isolated.
One major problem with this implementation is the fact that Schottky diodes are slow to react to undershoot voltages with fast edge rates. This could cause the N-channel pass transistor to turn on and allow a large amount of current to affect the isolated bus. The amount of current flowing from one bus to another will be significant because of an additional parasitic NPN transistor that is formed across transistor MN1. Since the base of the parasitic NPN is tied to the substrate and is at ground, the negative undershoot voltage will turn on the NPN transistor if not clamped below the threshold voltage. In addition, the large capacitance of the Schottky diodes needed for clamping an undershoot makes this alternative very undesirable in bus switch applications.
Another implementation to protect bus switches against undershoot voltages is that of a charge pump. A charge pump with a negative voltage output could be used to control the gate of the pass transistor and the substrate bias. This could keep both the N-channel pass transistor MN1 and the parasitic NPN off and keep the buses isolated. The problem with the charge pump solution is that the power supply current (Icc) is much higher, and the silicon area required on a die is greater due to the charge pump cells and the oscillator. Since bus switches are often found in systems where power consumption is critical, using a charge pump for undershoot voltage protection is not an effective solution.
A CMOS bus switch with undershoot protection to help prevent the corruption of data when the switch is open and the buses are isolated. The present invention includes an active pull-up clamp circuit one coupled to each bus. The clamp circuit does not use a charge-pump or Schottky diodes, and occupies reduced silicon wafer space.